Evidence that schizophrenia may involve infection by a virus (or viruses) has been indirect. The evidence includes the phenomenology of schizophrenia (insofar as it may be mimicked by some viral encephalitides), epidemiological factors (including a predominance among patients with late winter/early spring births, a north-south gradient, and occasional clustering of cases), and indirect laboratory evidence (gliosis in some neuropathological studies, spinal fluid protein abnormalities, and abnormalities in cell-mediated immunity). The discovery of the human retroviruses (HTLV-1, HTLV-11 and HIV, now also known to affect the central nervous system) and the development of new techniques in human retrovirology made it possible to investigate the role of this class of viruses in the etiology of schizophrenia. Cultures of peripheral lymphocytes of patients with chronic schizophrenia were established and tested for the retrovirus-specific enzyme reverse transcriptase.